In order to increase the efficiency and power output of gas turbines, there generally must be a concomitant increase in the operating temperatures and pressures of these engines. Unfortunately, as increased demands are placed upon existing materials systems they begin to fail thereby placing a practical limit on operating parameters.
Over the years various materials have been developed and utilized to overcome the previous difficulties and boost the performance of the engines.
In particular, complex oxide dispersion strengthened superalloys may be fabricated from powder made by the mechanical alloying process described in, for example, U.S. Pat. No. 3,591,362. These alloys have a complex composition, being strengthened with several solid solution additions, such as molybdenum, tungsten, rhenium, (niobium and tantalum to a lesser extent), gamma prime (Ni.sub.3 Al) for intermediate temperature strength and yttria (Y.sub.2 O.sub.3)for extreme elevated temperature creep and stress rupture resistance. These alloys are given elaborate heat treatments to develop both a high aspect ratio grain structure and the correct gamma prime structure. Alloys of this type are ideally suited, strengthwise, for airfoil section components in gas turbines and, in particular, to industrial gas turbines where long service life is a paramount consideration. However, as is common with almost all superalloys, these oxide dispersion strengthened superalloys are not satisfactorily resistant to hot corrosion for extreme temperatures and/or long service lives. This is especially true for those turbines operating in marine environments and/or consuming low grade fuels containing such contaminants as sulfur, vanadium, sodium chloride and organic phosphates.
A non-limiting example of a mechanically alloyed superalloy is INCONEL.RTM. alloy MA 6000 produced by assignee. INCONEL alloy MA 6000 is a mechanically alloyed nickel-base superalloy strengthened by both oxide dispersion strengthening and precipitation hardening for high creep and rupture properties at 1095.degree. C. (2000.degree. F.). Useful as it is because of good oxidation and sulfidation resistance, it was recognized that by increasing its resistance to corrosion attack at elevated temperatures, it would be ideal for turbine airfoil applications. A possible technique for increasing this (and any ODS) alloy is coating the material.
Various types of hot corrosion resistant coatings have been evaluated on these alloys to extend their service life. Aluminizing, low activity aluminizing and platinum aluminizing result in coatings that fail in relatively short times due to the formation of Kirkendall porosity at or immediately adjacent to the coating/ substrate interface. Kirkendall porosity leads to coating loss particularly under thermal cycle conditions as the voids coalesce into continuous cracks in the region of the maximum diffusion gradient. The Kirkendall porosity is the result of interdiffusion between the coating and the substrate. NiCoCrAlY(Ta) plasma spray and physical vapor deposition coatings have been found that are less prone to Kirkendall voiding than aluminide coatings. However, these coatings result in less than satisfactory service lives due primarily to the diffusion of aluminum from the coating into the substrate and the diffusion of nickel from the substrate into the coating.
A coating system is needed to assure long service life of oxide dispersion strengthened, gamma prime containing superalloys that will be exceptionally resistant to the formation of Kirkendall porosity at elevated operating temperatures. Such a coating system may be based on NiCoCrAlY(Ta) compositions provided that a means is found to inhibit the interdiffusion between the coating and the substrate. As far is as known, attempts to solve this interdiffusion problem have been unsuccessful. A diffusion barrier based on zirconium dioxide (ZrO.sub.2) results in severe limitations to operating conditions owing to the poor resistance to induced stresses of the coating system. In a different approach, U.S. Pat. No. 4,675,204 discloses melting a 0.3 to 0.5 mm layer of the substrate using a high energy beam in order to free this zone of yttria particles, which tend to act as nucleation sites for the vacancies resulting from interdiffusion. This concept is difficult to achieve on actual hardware owing to the complexity of the process and because the interdiffusion mechanism is not significantly inhibited by removing the yttria from the substrate.